When drilling bore holes through geological strata, in mining or exploration work, it is almost impossible to ensure that the bore hole is drilled out in the exact path intended.
The data obtained from such bore holes must therefore be interpreted in the light of possible variations in its paths from the intended path. Thus the information which can be gathered from for example a combination of several separate bore holes through a particular geological body or structure may be subject to wide differences in interpretation, depending upon the degree of wander of the various bore hole paths.
Many devices have been proposed to be passed down such bore holes, in an attempt to log deviations of the path. Such devices have included simple pendulum devices, and simple magnetic devices, and have included highly complex gyroscopic devices, and optical and photographic devices have also been proposed.
However, the conditions under which such devices are used make it absolutely essential that they shall be as far as possible totally independent of climatic conditions, magnetic variations in the strata, or any other conditions which might otherwise interfere with the operation of or accuracy of the instrument. In addition, many prior proposals for this type of instrument have involved the use of electrical cables being threaded down the bore hole. In many cases this is impractical. The instrument will not always fall readily down the hole, especially if it wanders substantially from the vertical. It must therefore be attached to a series of drill rods and progressively fed down the hole. This has made it extremely difficult to provide any control over the instrument, from the surface.
Accordingly, it is desirable to provide an instrument of this type which is totally automatically operated independently of the surface.